After logs are milled to form lumber, the lumber is usually stacked for drying. Typically, the lumber is stacked in layers separated by transversely extending spacers called "stickers". For example, each layer of a stack may consist of two inch by four inch beams (two by fours) laid side by side lengthwise. Layers in a stack could also consist of beams of other dimensions, boards, planks, or the like. Between each layer, one inch by two inch or other dimensioned stickers are laid crosswise to the two by fours to allow air to circulate around and dry the two by fours. Often, the stacks are located in a kiln or drying oven to provide a controlled, heated environment for more rapid drying.
Usually, the lumber on the outer sides of a stack is subjected to a greater circulation of dry air than is the lumber in the stack's center, resulting in the lumber on the outer sides drying faster. Also, the lumber in the same layer of a stack may vary significantly in its initial moisture content. Therefore, estimating the dryness of the lumber in the stack's center by measuring the moisture content of the lumber on the outer sides of the stack is usually inaccurate. To better regulate the drying process, it is frequently desirable to determine the moisture content of the lumber in the center of a stack of wood without having to remove lumber from the stack.
One type of moisture detector used to measure the moisture content of wood uses two pin-shaped electrodes which are driven a predetermined distance apart into a piece of wood. With the electrodes inserted into a piece of wood, the moisture detector applies a voltage across the two electrodes and measures the current flowing between the electrodes. The amount of current flowing between the electrodes for a given voltage difference between the electrodes is directly related to the moisture content of the wood. Thus, by properly scaling the measured amount of current flow, the moisture detector is able to determine the moisture content of wood.
There are several problems with using this type of moisture detector to determine the moisture content of wood in the center of a stack of wood. First, it is difficult to drive pin-shaped electrodes into the wood in the stack's center. One apparatus which attempts to overcome this limitation comprises a rod with a T-shaped handle at one end. A flange is mounted at an end of the rod opposite the handle and projects perpendicularly to the rod. Two pin-shaped electrodes are mounted to project perpendicularly from the flange approximately one and a half inches from the rod. The electrodes are connected to wires which run through a hollow center of the rod to a co-axial cable connector attached to the handle. A meter which provides a voltage to the electrodes and measures the current flow connects to the electrodes using the co-axial cable connector.
The rod and flange act as a lever arm for driving the electrodes into a piece of wood in the center of a stack. First, the rod is inserted between layers of the stack with the flange oriented horizontally and the electrodes pointing upwards. With the electrodes pointing towards a piece of wood in the layer of the stack above the device, the rod is rotated about its longitudinal axis using the handle. During rotation, the rod seats against the layer of the stack beneath the apparatus and the flange rotates upward. This moves the electrodes on the flange towards the wood and drives them into the wood.
A second problem with moisture measurement devices employing pin-shaped electrodes is that because the electrodes are driven into the wood, the wood is damaged when its moisture content is measured. A further problem is that it is difficult to measure the moisture deep within a piece of wood. To measure the moisture deeply within the wood, considerable force must be used to drive the electrodes deeply into the wood, resulting in even greater damage to the wood.
A second type of device for measuring the moisture content of wood uses an electromagnetic wave detection technique. One such device is the prior art Hand-Held Moisture Meter Model L601 (the L601 Meter) manufactured by Wagner Electronic Products. The L601 meter has a sensor comprising three electrodes: a detector electrode, a transmitter electrode, and a ground electrode. These electrodes are configured as flat conducting plates which are placed against or in close proximity to a piece of wood whose moisture content is to be measured.
The detector electrode is driven with a radio frequency (RF) excitation signal generated by an oscillator circuit to produce electromagnetic waves. When the electrodes are placed against or in close proximity to wood, the electromagnetic waves penetrate into the wood in a volume which is approximately 1 inch deep and 3 inches square. The electromagnetic waves induce an RF sensing signal in the detector electrode whose amplitude is related to the moisture content of the wood. The L601 Meter displays the amplitude of the RF sensing signal on an analog display calibrated to show the moisture content of the wood.
The L601 Meter and like electromagnetic wave detection devices eliminate the need for driving pin-shaped electrodes into wood to measure its moisture content. However, such devices are only effective for measuring wood which is in close proximity to the sensor.